The present invention relates to a silver halide emulsion having low fogging and high sensitivity and also relates to a silver halide color photographic light-sensitive material using the emulsion.
In recent years, the requirements for silver halide photographic emulsions are becoming severer. To speak more specifically, the emulsion is demanded to have higher sensitivity, improved relationship of the sensitivity/fog ratio and higher image quality. One of the techniques for elevating the sensitivity of a silver halide emulsion and thereby attaining a higher image quality is to use tabular grains. The tabular grain has advantage in that higher sensitivity can be achieved including the improvement in the color sensitization efficiency due to the sensitizing dye, the relationship of the sensitivity/granularity ratio can be improved, the sharpness can be elevated owing to the optical properties peculiar to the tabular grain, and the covering power can be increased.
However, the silver halide tabular grain capable of successfully ensuring higher sensitivity and improved relationship of the sensitivity/granularity ratio suffers from increased fogging. Thus, the problem to be solved is to attain low fogging at the same time.
With respect to the technique for improving the relationship of the sensitivity/granularity ratio of tabular grains, for example, U.S. Pat. Nos. 5,219,720 and 5,334,495 disclose a technique of using tabular grains having a small spacing between twin planes. Even by this technique, however, it has been found to be impossible to satisfy two requirements at the same time that the tabular grain has a high aspect ratio and a small twin plane spacing and that the grain shape/structure (twin plane spacing distribution, tabular grain ratio and equivalent-circle corresponding diameter distribution (i.e., equivalent-circle diameter distribution)) is highly uniform among grains. Thus, the above-described technique is not enough to obtain a silver halide emulsion having both low fogging and high sensitivity.
With respect to the technique for controlling the shape of silver halide grains and thereby forming uniform silver halide grains, for example, U.S. Pat. Nos. 5,580,712 and 5,670,616 disclose a technique of using a biopolymer at the grain formation. However, these patents neither teach nor suggest the technique of the present invention where a dispersion medium having a low viscosity is used at the grain formation, so that the grains formed can have a high aspect ratio and a small twin plane spacing and among the grains, the twin plane spacing distribution, the tabular grain ratio and the equivalent-circle diameter distribution can be uniform.
Conventional gelatin heretofore used in silver halide emulsions is described in detail below. In the process of forming silver halide emulsion grains, gelatin is used in several steps. The function required for the gelatin differs among respective steps and the gelatin is preferably designed in the molecular level to satisfy the use purpose in each step.
During the process of forming silver halide emulsion grains, at least one gelatin is used as a protective colloid of a silver halide grain in the nucleation step and subsequent ripening/growing step. Recently, for preparing a silver halide tabular grain emulsion having a high aspect ratio, a chemically modified gelatin is being aggressively used. JP-B-5-12696 (the term xe2x80x9cJP-Bxe2x80x9d as used herein means an xe2x80x9cexamined Japanese patent publicationxe2x80x9d) discloses a technique of using a gelatin of which sulfide group is rendered ineffective by hydrogen peroxide or the like, as the protective colloid and thereby preparing tabular grains having a small thickness. Also, JP-A-8-82883 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) discloses a technique of making the amino group and the sulfide group ineffective and thereby preparing tabular grains having a small thickness. In addition, JP-A-10-148897 discloses a technique of introducing two or more carboxyl groups at the time of chemically modifying the amino group in gelatin and thereby preparing monodisperse tabular grains having a small thickness. Furthermore, U.S. Pat. No. 5,580,712 and EP-A-926544 disclose a technique for increasing the formation ratio of tabular grains, where gelatin for use as a protective colloid is designed in the molecular level and a gelatin derivative prepared using a technique of chemical synthesis or genetic engineering is used.
However, there is not known a technique such that gelatin for use as a protective colloid is subjected to chemical modification or molecular designing during the grain formation process of a silver halide grain emulsion with the intention of changing the property, thereby optimizing the macroscopically physical properties of the dispersion medium, such as viscosity, and in turn obtaining objective silver halide grains. For example, on taking account of the dependency of the solubility of silver halide on the temperature, it would be easily anticipated that if gelatin incapable of gelling under a lower temperature can be used, the silver halide grains may be more finely formed.
At present, the industrially produced gelatin is generally derived from collagen contained in animal bone or skin. One of the defects of the animal collagen-derived gelatin is the polydisperse molecular weight. This polydisperse molecular weight is apparently disadvantageous for controlling the physical properties of a dispersion medium containing the gelatin. If a gelatin having a monodisperse molecular weight is found, the grain shape may also be rendered monodisperse in the process of forming silver halide grains.
According to the technique of producing a gelatin derivative using a technique of chemical synthesis or genetic engineering described in U.S. Pat. No. 5,580,712 and EP-A-926543, the gelatin derivative theoretically has a monodisperse molecular weight (also experimentally, when the molecular weight distribution is measured using a gel permeation chromatography, the molecular weight is found to be substantially monodisperse). However, this gelatin having a monodisperse molecular weight is not intended to use so as to optimize the physical properties of the dispersion medium used in the production process of a silver halide grain emulsion.
With respect to the technique for forming silver halide fine grains, for example, JP-A-10-43570, JP-A-4-292416, U.S. Pat. No. 5,250,403, JP-W-6-507255 (the term xe2x80x9cJP-Wxe2x80x9d as used herein means an xe2x80x9cunexamined published international patent applicationxe2x80x9d), JP-A-4-139440 and JP-A-9-179225 disclose it. These patents also disclose a technique of using silver halide grains in the growth step and thereby forming silver halide tabular grains.
It is known that when silver halide tabular grains are formed by using silver halide fine grains in the growth step, tabular grains having a very small thickness are obtained and the grains are uniform in the halogen composition.
In the case of using silver halide fine grains in the growth step, the fine grains added are dissolved and consumed for the growth of previously existing tabular grains. However, if the fine grain has a large sphere-corresponding diameter (i.e., a large equivalent-sphere diameter), these coarse grains are not dissolved and remain even after the growth step. If the fine grain added has a twin plane, the grain itself grows and turns to a silver halide tabular grain, as a result, the grain size or grain size distribution of finally obtained grains cannot be controlled.
In order not to allow the fine grains to remain, a method where a silver salt aqueous solution and a halide salt aqueous solution each in a low concentration are mixed in an external mixer to form silver halide fine grains and the grains are immediately used in the growth step, may be used. By reducing the concentrations of the solutions added, the grain size of silver halide fine grains can be made small. This method has, however, a problem in the productivity.
In another method for not allowing fine grains to remain, a part of the internal solution used for the growth is re-circulated into the external mixer. This method has, however, disadvantage in that the apparatus is complicated.
Under these circumstances, one of the important problems to be solved in the field of forming photographic silver halide grains is to form monodisperse silver halide grains having a small grain size while preventing the apparatus from being complicated and while ensuring sufficiently high productivity.
One object of the present invention is to provide a silver halide emulsion having low fog and high sensitivity.
Another object of the present invention is to provide a silver halide color photographic light-sensitive material using the emulsion.
As a result of extensive investigations, the present inventors have found that when a dispersion medium having a low viscosity is used in the grain formation process, tabular grains having a high aspect ratio and a small twin plane spacing, in which the twin plane spacing distribution, the tabular grain ratio and the equivalent-circle diameter distribution are uniform among grains, can be obtained and the capability of the silver halide emulsion can be remarkably improved (reduction in fogging and elevation of sensitivity).
The above-described objects of the present invention can be effectively attained by the following embodiments.
(1) A silver photographic emulsion comprising silver halide grains, which is obtained through a process including at least one step of forming grains using a dispersion medium having a low viscosity, wherein 50% or more of the entire projected area of all silver halide grains in the emulsion is occupied by silver halide tabular grains having an aspect ratio of 4 or more and a twin plane spacing of 0.012 xcexcm or less.
(2) The silver halide photographic emulsion as described in (1) above, which is obtained through a process including at least one step of forming grains using a dispersion medium having a low viscosity at a low temperature.
(3) The silver halide photographic emulsion as described in (1) or (2) above, wherein the protective colloid in the dispersion medium having a low viscosity is gelatin.
(4) The silver halide photographic emulsion as described in any one of (1) to (3) above, wherein the protective colloid in the dispersion medium has a monodisperse molecular weight distribution.
(5) The silver halide photographic emulsion as described in any one of (1) to (4) above, wherein the protective colloid in the dispersion medium is produced using a technique of chemical synthesis or genetic engineering.
(6) The silver halide photographic emulsion as described in (1) above, wherein 50% or more of the entire projected area of all silver halide grains in the emulsion is occupied by silver halide tabular grains having an aspect ratio of 4 or more and a twin plane spacing of 0.010 xcexcm or less.
(7) The silver halide photographic emulsion as described in (1) above, wherein 50% or more of the entire projected area of all silver halide grains in the emulsion is occupied by silver halide tabular grains having an aspect ratio of 8 or more and a twin plane spacing of 0.012 xcexcm or less.
(8) The silver halide photographic emulsion as described in (1) above, wherein 50% or more of the entire projected area of all silver halide grains in the emulsion is occupied by silver halide tabular grains having an aspect ratio of 8 or more and a twin plane spacing of 0.010 xcexcm or less.
(9) The silver halide photographic emulsion as described in any one of (1) to (8) above, wherein the coefficient of variation in the twin plane spacing distribution of all silver halide grains in the emulsion is from 3 to 25%.
(10) The silver halide photographic emulsion as described in any one of (1) to (8) above, wherein the coefficient of variation in the twin plane spacing distribution of all silver halide grains in the emulsion is from 3 to 15%.
(11) The silver halide photographic emulsion as described in any one of (1) to (10) above, wherein from 95 to 100% of the entire projected area of all silver halide grains in the emulsion is occupied by tabular grains having an aspect ratio of 4 or more and a twin plane spacing of 0.012 xcexcm or less.
(12) The silver halide photographic emulsion as described in any one of (1) to (11) above, wherein the coefficient of variation in the equivalent-circle diameter distribution of all silver halide grains in the emulsion is from 3 to 20%.
(13) The silver halide photographic emulsion as described in (1) to (12) above, which is obtained through a process including at least one step of forming grains using a dispersion medium having a low viscosity and which is prepared using a silver halide fine grain emulsion satisfying all the following requirements (i) to (iv):
(i) the silver bromide content is 50 mol % or more;
(ii) the average equivalent-sphere diameter is 0.02 xcexcm or less;
(iii) the coefficient of variation of the equivalent-sphere diameter is 30% or less; and
(iv) the ratio by number of grains containing a twin plane is 3% or less.
(14) A silver halide color photographic light-sensitive material comprising a support having thereon at least one silver halide emulsion layer, wherein at least one of the silver halide emulsion layers contains the silver halide photographic emulsion described in any one of (1) to (13) above.